An iPhone camera feature starting with the iPhone 6s that records three seconds of video (MOV) along with the still JPEG photo. When the picture is taken, the previous 1.5 seconds of the continuously recorded video are retained along with the subsequent 1.5 seconds. The files are double the size of the normal 2MB JPEG.
When Live Photos is selected, the camera begins taking video, which means that users should compose the picture a couple seconds before it is actually snapped and not say anything unless they want that audio in the Live Photo afterwards. All video before the 1.5 second lead-in is discarded when the shutter is snapped. See iPhone 6s
and Force Touch